[steveklabnik]

> Without giving it much thought, why is this hard for a modern C++ compiler to detect?

It is hard in the general case because none of the operations individually are a problem: "create a variable on the stack", "create a pointer to a variable on the stack", and "return a pointer" are all valid operations. It's the last two together that become a problem. And there's no mechanism in C++ to connect the two together, so that this case can be recognized and protected against. Compilers can try, and see some obvious cases, like you've noticed, but it's going to be a conservative analysis at best.

> Is it an intractable issue?

In some sense, yes, in others, no. Solving it correctly in 100% of cases is not going to happen. But they're working on some things that will hopefully catch most cases. We are supposed to get some more news on that soon, we'll see.